The present invention relates generally to the field of scanning devices. In particular, the present invention relates to the profiling of a component which includes scanning a spot of light through a range of angles. More specifically, the present invention relates to the measuring of the external surface profile of a component using a non-contact optical technique. Even more specifically, the present invention relates to the measuring of the external surface profile of a component using a non-contact optical technique which scans the field of view by utilizing a rotating mirror and which precisely determines the angular position of the spot of light during scanning.
The present invention describes several highly accurate methods for determining the pointing angle of a laser beam. The theory and use of these inventions are introduced by examining how these inventions aid the construction and use of a non-contact laser scanning system. A body of useful prior art for this work is described in U.S. Pat. No. 6,441,908, issued to Johnston et al.
We have developed an instrument where some form of light illuminates a spot on the surface of an object to be measured. The light source is usually a laser beam and the beam is usually first reflected off a mirror, as shown in FIG. 1. By repositioning the mirror, usually using a rotational motion, we can reposition the light spot to measure a series of locations along the surface of the object. The challenge is to determine, very accurately, what the position of the mirror is so we can accurately determine the position of the laser (ideally to within 50 xcexcrad) and ultimately determine the position of the laser on the object.
Mechanical Position Method
One class of previous techniques for determining the pointing angle of a laser have required measuring the mechanical deflection of the mirror using techniques that include the use of rotary encoders and specialized potentiometers. The laser deflection can then be inferred using the fact that
xcex8Optical=2xcex8Mechanical.xe2x80x83xe2x80x83(0)
Due to noise, jitter and limited resolution, these mechanical deflection methods typically do not suffice for measuring beam position down to 50 xcexcrad.
Constant Velocity Method
A potentially more useful technique for determining the pointing angle of the laser is to make optical measurements of the laser beam as it is swept over a range of angles and then infer the position using angular velocity. As shown in FIG. 2, the laser first sweeps across an initial position sensor (Tick). This event latches a high precision counter, resulting in an initial timer value (TTick). The timer is also latched for each ith desired measurement of the laser beam position (Ti). Finally, as the laser crosses the end of scan position sensor (Tock), the timer is again latched (TTock). These sensors can be any photo-detector but it is assumed in this disclosure that the high resolution bi-cell optical trigger configuration is used.
If the angular distance between Tick and Tock (xcex94xcex8) are known (i.e. fixed or predetermined) and we assume a constant angular velocity, then it is possible to calculate a high precision angular position value for the ith position measurement (xcex8i) using                               θ          i                =                              θ            0                    +                                    ω              0                        ⁢                          t              i                                +                                    1              2                        ⁢            α            ⁢                          xe2x80x83                        ⁢                          t              i              2                                                          (        1        )            
where xcfx890 is the initial angular velocity and xcex1 is the (constant) angular acceleration. First, we arbitrarily set xcex80=xcex8Tick=0. This reduces (1) to                               θ          i                =                                            ω              0                        ⁢                          t              i                                +                                    1              2                        ⁢            α            ⁢                          xe2x80x83                        ⁢                                          t                i                2                            .                                                          (        2        )            
The average value of xcfx89 can be found using                               ⟨          ω          ⟩                =                              Δθ                          Δ              ⁢                              xe2x80x83                            ⁢              T                                =                                                                      θ                  Tock                                -                                  θ                  Tick                                                                              T                  Tock                                -                                  T                  Tick                                                      .                                              (        3        )            
Assuming no angular acceleration, e.g. xcex1=0, the ith angular position (xcex8i) relative to xcex8Tick can be determined after the sweep is complete by substituting  less than xcfx89 greater than  for xcfx890 and using                               θ          i                =                                            ⟨              ω              ⟩                        ⁢                          T              i                                =                                                                      θ                  Tock                                -                                  θ                  Tick                                                                              T                  Tock                                -                                  T                  Tick                                                      ⁢                                          T                i                            .                                                          (        4        )            
This optical angular velocity technique can suffice for measuring beam position down to 50 xcexcrad as long as there is no angular acceleration and the counter is of sufficiently high resolution with low jitter in the triggering/latching circuits. This technique has the advantage of not requiring a specific value of xcfx89 as long xcfx89 remains constant during the sweep so that at any time the instantaneous angular velocity ((xcfx89i) approximates the average velocity,
xcfx89i≈ less than xcfx89 greater than .xe2x80x83xe2x80x83(5)
Constant Acceleration Method
The constant velocity method can fail when there is sufficient angular acceleration present between Tick and Tock to invalidate (5). In the special case where xcex1 is slowly varying, such as a sine wave, then the value of xcex1 can be approximated as constant between Tick and Tock. In such a case, knowledge of the initial angular velocity (xcfx89Tick) and final angular velocity (xcfx89tock) suffice to determine the laser position with sufficient accuracy. These new angular velocity values can be measured using a variation of the configuration as shown in FIG. 3. A pair of Tick trigger sensors separated a known distance (xcex94xcex8Tick) is provided, as well as a corresponding pair of Tock sensors.
The ith angular position can now be found using                               θ          i                =                                            ω              0                        ⁢                          T              i                                +                                    1              2                        ⁢                          ⟨              α              ⟩                        ⁢                          T              i              2                        ⁢                          xe2x80x83                        ⁢            where                                              (        6        )                                          ⟨          α          ⟩                =                                                            ω                Tock                            -                              ω                Tick                                                    Δ              ⁢                              xe2x80x83                            ⁢                              T                Tock                                              =                                                                      ω                  Tock                                -                                  ω                  Tick                                                                              T                  Tock                                -                                  T                  Tick                                                      ⁢                          T              i                        ⁢                          xe2x80x83                        ⁢            and                                              (        7        )                                                      ω            0                    =                                    ω              Tick                        =                                                            θ                  Tick2                                -                                  θ                  Tick1                                                                              T                  Tick2                                -                                  T                  Tick1                                                                    ,                  xe2x80x83                ⁢                              ω            Tock                    =                                                                      θ                  Tock2                                -                                  θ                  Tock1                                                                              T                  Tock2                                -                                  T                  Tock1                                                      .                                              (        8        )            
Challenge of Instantaneous Acceleration
The constant velocity and constant acceleration methods of determining the angular position of the beam using optical measurements suffice for xe2x80x9cwell behavedxe2x80x9d motion profiles of the rotating deflection mirror, e.g. where there is no appreciable instantaneous acceleration. However, when xcex1 is not constant between Tick and Tock, large errors in the determined value of xcex8i can result. Such a case can result if a resonant device such as a torsional pendulum structure rotates the mirror. This is a case where both xcex1 and xcfx89 are described by sine waves with periods of approximately twice the time between Tick and Tock. If the structure was tuned so that the motion was just sufficient for the beam to cross both Tick and Tock, then xcfx89Tick≈xcfx89Tock=0 and (6) clearly breaks down. A similar extreme case is simulated in FIGS. 4a and 4b where a mirror rotating with a base velocity of xcfx890=1.0 RPS has a 6 Hz sinusoidal velocity variation with amplitude of 0.5 RPS. The solid line in FIG. 4a shows the actual velocity and the dashed line shows the velocity calculated using (3). The dashed line in FIG. 4b shows the expected position of the mirror using (4), which is significantly different than the actual position shown as a solid line, up to 10 degrees in the center of the sweep. FIG. 4b would be identical if (6) were used because the initial and final velocities are the same.
Another case where (6) breaks down is when an instantaneous acceleration xe2x80x9ceventxe2x80x9d perturbs an otherwise constant motion profile. This can occur many ways, such as when an impulse from nearby equipment causes a vibration event or when the whole system is in motion in a fashion that couples with the angular momentum of the mirror or, even more subtle, when the bearings bind or drop into a xe2x80x9cgroovexe2x80x9d in their races.
Acquiring and Using Angular Motion Data
The preceding discussion illustrates that a more refined method is needed to profile the acceleration, velocity or position profile between the Tick and Tock sensors. Properly accomplished, enough motion data between Tick and Tock would allow breaking xcex94xcex8 into sufficiently small angular ranges where the acceleration can be considered constant. In practice, acquiring this data can be difficult. Examination of FIG. 2 shows that, unless a transparent sensor is used, the Tick and Tock sensors block the outgoing beam at the beginning and end of the beam sweep. Any subsequent motion data between Tick and Tock must be acquired without fully blocking the outgoing measurement beam.
The several methods for gathering sufficient motion data between Tick and Tock that are described in this disclosure can be organized into the categories of:
Mechanical measurement techniques directly coupled to the motion of the deflection mirror. These techniques generally provide noisy, low-resolution data available as a constant stream of measurements between Tick and Tock;
Optical techniques where a portion of the outgoing measurement beam is sampled and measure to provide motion data. The techniques described here are typically high resolution, low jitter data available as a small set of discreet samples spanning between Tick and Tock; and
Optical techniques where a separate beam is directed off the deflection mirror and measured to provide motion data. The separate beam can be from a separate source or sampled off the input beam before the deflection mirror.
These and other advantages of the present invention will become more fully apparent from the detailed description of the invention hereinbelow.
The present invention is directed to a system for high-precision determination of the angular position of a light beam in an optical scanning device, the system comprising: a source of light that emits a substantially collimated primary light beam; a light-sensitive sensor; an optical element used to focus an image onto the sensor; and a rotatable mirror system that re-directs the primary light beam to a plurality of locations of an external surface of a component, the rotatable mirror system comprising: a rotary axis; a mirror structure that rotates about the rotary axis; at least one detector positioned to intercept a light beam that has been reflected off the mirror structure at an angle representing a start-of-scan such that a first trigger pulse is generated, and positioned to intercept a light beam that has been reflected off the mirror structure at an angle representing an end-of-scan such that a second trigger pulse is generated; and an auxiliary system that provides measurement data between the start-of-scan angle and the end-of-scan angle; wherein the first trigger pulse and the measurement data are used to determine the angular position of the primary light beam.
In one embodiment, the auxiliary system includes a measurement device which is coupled to the rotary axis, and wherein the first and second trigger pulses and the measurement data are all used to determine the angular position of the primary light beam.
In another embodiment, the auxiliary system includes an auxiliary light-sensitive sensor and a beam splitter, wherein the beam splitter is positioned to intercept the primary light beam subsequent to reflecting off of the mirror structure, and wherein the beam splitter re-directs a portion of the primary light beam onto the auxiliary sensor.
In yet another embodiment, the auxiliary system includes an auxiliary light-sensitive sensor and a beam splitter, wherein the beam splitter is positioned to intercept the primary light beam prior to reflecting off of the mirror structure, and wherein the beam splitter re-directs a portion of the primary light beam onto the mirror structure which reflects the re-directed primary light beam portion onto the auxiliary sensor.
In still yet another embodiment, the auxiliary system includes an auxiliary light-sensitive sensor and an auxiliary light source, wherein the auxiliary light source emits a substantially collimated auxiliary light beam onto the mirror structure which reflects the auxiliary light beam onto the auxiliary sensor.